Inkjet print heads for generating and expelling droplets of fluid are well known in the art. A number of actuation methods are known to be employed in such print heads. In a known inkjet print head, a piezo stack, comprising a first electrode, a second electrode and a piezo-material layer therebetween, is driven to deform a flexible wall of a pressure chamber such that a pressure wave is generated in a fluid present in the pressure chamber. The pressure chamber is in fluid communication with a nozzle orifice of the print head and the pressure wave is such that a droplet of the fluid is expelled through the nozzle orifice.
In order to actuate, a drive voltage is applied to the piezo stack, which piezo stack acts as a capacitor. Suitable drive circuitry supplies an actuation voltage and corresponding current. In order to generate and supply such drive voltage and current, power is consumed and heat is generated in the drive circuitry. In present inkjet print heads made using semiconductor technology (micro electromechanical systems (MEMS) technology) a high density arrangement of nozzle orifices and corresponding actuators is obtainable. However, in such high density arrangements and operating at a high frequency, a relatively large amount of heat is generated in the drive circuitry, including in any electrodes in the inkjet print head. A density of an arrangement of electrodes and a cross-section of each electrode (determining an electrical resistance in the electrodes) becomes limited due to which the design of such print heads becomes limited. Further, due to heat generation in the voltage generating circuitry, incorporating the voltage generating circuitry in the inkjet print head is not feasible. It is advantageous to have a print head design in which a relatively low amount of heat is generated. Such a design is disclosed in WO2015/010985, for example.
The disclosed inkjet print head comprises a fluid channel for holding a channel amount of fluid. The fluid channel comprises a pressure chamber in fluid communication with the nozzle orifice. The inkjet print head further comprises a piezo actuator. The piezo actuator comprises an active piezo stack and a membrane. The active piezo stack comprises a first electrode, a second electrode, and a piezo-material layer arranged between the first and the second electrode. The active piezo stack is provided at a surface of a membrane, which membrane forms a flexible wall of the pressure chamber.
It is noted that it is common that the active piezo stack is arranged opposite to the pressure chamber, but it is contemplated that, in an embodiment, the active piezo stack may be arranged at a pressure chamber side of the membrane.
As used herein, the flexible wall is a wall or part of a wall of the pressure chamber which wall or part of the wall is enabled to bend. Hence, a wall dimension of the membrane forming the flexible wall, in particular length and width of the flexible wall, may be determined by dimensions of the pressure chamber, but may as well be determined by other structural elements.
The fluid channel, when holding the channel amount of fluid, has a fluid channel compliance and the piezo actuator has an actuator compliance. The fluid channel compliance has a number of contributions, inter alia from a compliance resulting from the amount of fluid present and a compliance resulting from the print head structure, including the compliance of the materials used. It is noted that the actuator compliance is not included in the fluid channel compliance; adding the actuator compliance and the fluid channel compliance results in a total system compliance or, in other words, the fluid channel compliance corresponds to the total system compliance minus the actuator compliance. In accordance with the present invention, the actuator compliance is larger than the fluid channel compliance. Preferably, the actuator compliance is significantly—e.g. 2, 3, 5, 10 or even more times—larger than the fluid channel compliance. Such a design is thus sensitive to actual compliances of certain parts of the print head.
In more detail and as disclosed in WO2015/010985, an acoustic design of a piezo-actuated inkjet print head is inter alia defined by an unloaded volume displacement of the actuator in response to a drive voltage and by the total system compliance. Such acoustic design determines the droplet generation, including a droplet generation frequency. When designing an inkjet print head and starting from print head requirements, an acoustic design may be selected. Then, in order to optimize an energy consumption without affecting the acoustic design, a ratio between the fluid channel compliance and the actuator compliance may be selected, provided that the total system compliance fits the acoustic design. As is described in more detail hereinbelow in relation to FIG. 2, an energy coupling coefficient indicating an energy efficiency of the print head acoustics, i.e. the droplet forming process, compared to the electrical energy input, is defined by
                              ECC          acoustics                =                              k            2                    ⁢                                    B              act                                                      B                act                            +                              B                chan                                                                        (                  Eq          .                                          ⁢          1                )            
Energy efficiency is improved if the energy coupling coefficient ECC is increased. Based on Eq. 1, it is apparent that the energy coupling coefficient ECCacoustics of the print head acoustics is increased when the actuator compliance Bact is selected to be higher than the fluid channel compliance Bchan. The term k2 is an actuator energy coupling coefficient that has a certain optimal value. Based on such optimal value, the actuator compliance Bact may be deemed defined. Therefore, in practice, it may be considered that designing the inkjet print head to have a relatively low fluid channel compliance compared to the actuator compliance is a well suited method for improving the energy efficiency. Using a relatively low fluid channel compliance, an energy coupling coefficient will be relatively high and consequently, an overall energy efficiency of the print head is improved. As a consequence, a low driving voltage/low current may be used for driving the print head and thus power dissipation in the drive circuitry is decreased.
As the actuator compliance is a major contributor in the total system compliance, which has a significant contribution in defining the print head design, the actuator compliance is an important aspect to be accurately realized in an actual print head.
In practice, however, a manufacturing accuracy of a large number of features influences the resulting actuator compliance and defining manufacturing tolerances for each of such features may result in very strict tolerances that increase the costs for the print head manufacturing significantly or would even prohibit manufacturing as such strict tolerances may not be feasible. Therefore, in prior art, the inkjet print heads are manufactured in large quantities using not so strict tolerances. Then, the actuator compliance of the resulting print heads may be determined. In many instances the inaccuracies in the manufacturing compensate each other resulting in a sufficient number of print heads meeting the requirements on actuator compliance. Discarding of the print heads that do not have an actuator compliance within a desired actuator compliance range may thus be more cost effective and realistic than posing very strict manufacturing accuracies. Still, discarding of assembled print heads results in unnecessary costs and significantly reduced profits.
It is therefore an object of the present invention to increase a manufacturing yield of inkjet print heads of the above described type.